diff --git a/runtime/src/main/java/org/capnproto/WireHelpers.java b/runtime/src/main/java/org/capnproto/WireHelpers.java
index 8d85160..9327386 100644
--- a/runtime/src/main/java/org/capnproto/WireHelpers.java
+++ b/runtime/src/main/java/org/capnproto/WireHelpers.java
@@ -1035,7 +1035,7 @@ final class WireHelpers {
     };
 
     static SegmentBuilder setListPointer(SegmentBuilder segment, CapTableBuilder capTable, int refOffset, ListReader value) {
-        int totalSize = roundBitsUpToWords(value.elementCount * value.step);
+        int totalSize = roundBitsUpToWords((long) value.elementCount * value.step);
 
         if (value.step <= Constants.BITS_PER_WORD) {
             //# List of non-structs.